Sterilization effectiveness of in-duct ultraviolet germicidal irradiation system in liquid desiccant and indirect/direct evaporative cooling-assisted 100% outdoor air system

Microorganism contamination in air conditioning systems can cause respiratory diseases in building occupants. This study aims to evaluate and predict the performance of an in-duct ultraviolet germicidal irradiation (IDUVGI) system to reduce the microorganisms that can grow in the liquid desiccant and indirect/direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS). Bacteria and mold on the direct evaporative cooling (DEC) element and in the outdoor and supply air were monitored using a swab and Petrifilm for surfaces and impact sampler for air during the operation of the LD-IDECOAS. Microbial contamination was first verified in the air before performing the sterilization experiment. The experimental results showed that the airborne bacteria were reduced by 78.3% after the operation of the ID-UVGI system but there was almost no reduction in the airborne mold. The bacteria and mold concentration on the DEC surface were decreased by 99.7% and 96.7% respectively 3 h after the ID-UVGI operation. Even after the ID-UVGI system was operated for 6 days, the microbial contamination in the downstream water was not decreased significantly; the inside of the DEC element was suspected to be improperly sterilized. The operating times of the ID-UVGI system needed to inactivate the microorganisms were predicted from the UV-C intensity distribution by irradiation analysis. Legionella sp. and Penicillium sp. were predicted to be inactivated by 99.9% with UV-C exposures of 6 s and 15 min 57 s or less respectively when the LD-IDECOAS is operated at the highest air flow rate.